Just what is a thyristor?

A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor elements, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their operating status. Therefore, thyristors are commonly used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.

The graphical symbol of the silicon-controlled rectifier is normally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The operating condition in the thyristor is that each time a forward voltage is used, the gate will need to have a trigger current.

Characteristics of thyristor

1. Forward blocking

As shown in Figure a above, when an ahead voltage is used involving the anode and cathode (the anode is linked to the favorable pole in the power supply, as well as the cathode is linked to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), as well as the indicator light will not illuminate. This shows that the thyristor will not be conducting and has forward blocking capability.

2. Controllable conduction

As shown in Figure b above, when K is closed, along with a forward voltage is used for the control electrode (called a trigger, as well as the applied voltage is referred to as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.

3. Continuous conduction

As shown in Figure c above, right after the thyristor is switched on, even when the voltage on the control electrode is taken off (that is, K is switched on again), the indicator light still glows. This shows that the thyristor can still conduct. At the moment, in order to stop the conductive thyristor, the power supply Ea must be stop or reversed.

4. Reverse blocking

As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used involving the anode and cathode, as well as the indicator light will not illuminate currently. This shows that the thyristor will not be conducting and can reverse blocking.

5. In conclusion

1) If the thyristor is exposed to a reverse anode voltage, the thyristor is within a reverse blocking state regardless of what voltage the gate is exposed to.

2) If the thyristor is exposed to a forward anode voltage, the thyristor will only conduct once the gate is exposed to a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.

3) If the thyristor is switched on, as long as you will find a specific forward anode voltage, the thyristor will always be switched on no matter the gate voltage. Which is, right after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.

4) If the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.

5) The problem for that thyristor to conduct is that a forward voltage needs to be applied involving the anode as well as the cathode, plus an appropriate forward voltage also need to be applied involving the gate as well as the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode must be stop, or perhaps the voltage must be reversed.

Working principle of thyristor

A thyristor is actually an exclusive triode composed of three PN junctions. It could be equivalently thought to be composed of a PNP transistor (BG2) plus an NPN transistor (BG1).

1. In case a forward voltage is used involving the anode and cathode in the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. In case a forward voltage is used for the control electrode currently, BG1 is triggered to generate basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be introduced the collector of BG2. This current is delivered to BG1 for amplification then delivered to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, that is, the anode and cathode in the thyristor (the dimensions of the current is in fact dependant on the dimensions of the load and the dimensions of Ea), therefore the thyristor is totally switched on. This conduction process is done in a very limited time.
2. Following the thyristor is switched on, its conductive state is going to be maintained through the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it really is still inside the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to turn on. After the thyristor is switched on, the control electrode loses its function.
3. The best way to shut off the turned-on thyristor is always to decrease the anode current that it is insufficient to keep the positive feedback process. The way to decrease the anode current is always to stop the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor inside the conducting state is referred to as the holding current in the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor could be turned off.

What exactly is the difference between a transistor along with a thyristor?

Structure

Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.

The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.

Functioning conditions:

The task of the transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.

The thyristor requires a forward voltage along with a trigger current on the gate to turn on or off.

Application areas

Transistors are commonly used in amplification, switches, oscillators, and other elements of electronic circuits.

Thyristors are mostly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.

Means of working

The transistor controls the collector current by holding the base current to achieve current amplification.

The thyristor is switched on or off by manipulating the trigger voltage in the control electrode to realize the switching function.

Circuit parameters

The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.

To sum up, although transistors and thyristors may be used in similar applications in some instances, because of their different structures and operating principles, they may have noticeable differences in performance and use occasions.

Application scope of thyristor

• In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
• In the lighting field, thyristors may be used in dimmers and light-weight control devices.
• In induction cookers and electric water heaters, thyristors could be used to control the current flow for the heating element.
• In electric vehicles, transistors may be used in motor controllers.

Supplier

PDDN Photoelectron Technology Co., Ltd is an excellent thyristor supplier. It is one in the leading enterprises in the Home Accessory & Solar Power System, that is fully working in the development of power industry, intelligent operation and maintenance management of power plants, solar power panel and related solar products manufacturing.

It accepts payment via Bank Card, T/T, West Union and Paypal. PDDN will ship the goods to customers overseas through FedEx, DHL, by air, or by sea. If you are searching for high-quality thyristor, please feel free to contact us and send an inquiry.